P.either should fail with fatal error, if second parser failed with fatal, or should it?

Take a look at the following test (which currently fails):
```typescript
const fatalParser: P.Parser = i => error(i, ['expected'], true)
const parser5 = P.either(C.char('a'), () => fatalParser)
assert.deepStrictEqual(S.run('c')(parser5), error(stream(['c']), ['"a"', 'expected'], true))
```

I believe that the semigroup of ParserResult should use `getLastSemigroup` on the fatal property, and not `getFirstSemigroup`. I guess it is kind of a philosophical question as one can argue that I'm mistaken....

@gcanti thoughts?

If you think this should indeed be fixed, I have a PR ready to go